1. Field of the Invention
The present invention relates to a continuous-cast low-carbon resulfurized free-cutting steel, particularly to a continuous-cast low-carbon resulfurized free-cutting steel containing comparatively finely distributed manganese sulfide-base inclusions having a high plastic-deformability and thereby giving an excellent machined surface roughness.
2. Description of the Related Art
The increasing automatization and numerical-control of machining has doubled the demand for free-cutting steel over the last ten years. In this connection, serious efforts have been made to enhance the life of tools used in the cutting of free-cutting steel produced by continuous casting to the same level as that of tools used to cut free-cutting steel produced by ingot mold casting. For example, Japanese Examined Patent Publication (Kokoku) No. 59-19182 discloses that manganese sulfide in the form of a oval with a smaller length-to-width ratio effectively improves tool life. As a practical method of distributing manganese sulfide in the form of an oval in steel, this publication proposes limiting the %[S], %[C], and %[O ] in molten steel. Japanese Unexamined Patent Publication No. 59-205453 describes an effective method of improving tool life whereby manganese sulfide is spheroidized so as to have a length-to-width ratio of 5 or less and the content of Al.sub.2 O.sub.3 -base inclusion, which has a significant abrasive effect, is reduced. The publication proposes the addition of Te, Pb, and Bi for spheroidizing the manganese sulfide, and a content of Al in the steel of not more than 0.002%, to reduce the amount of Al.sub.2 O.sub.3. Further, among papers to academic societies, Tetsu-To-Hagane (Journal of The Iron and Steel Institute of Japan), vol. 71, 1985, No. 5, P. 242, for example, reports that, as the width of manganese sulfide increases, the tool wear is reduced, which coincides with the disclosures of the above-mentioned patent publications.
Nevertheless, although the tool life has been improved as mentioned above, the machined surface roughness of the continuous-cast free-cutting steel is still inferior to that of the ingot-mold-cast free-cutting steel, and thus is evaluated as being of a lower level. Therefore, a further improvement is desired.
A current prevailing theory for the mechanism through which manganese sulfide extends the tool life, and consequently improves the machined surface roughness, asserts that manganese sulfide reduces the shear stress on the shear plane. That is, the temperature at the tool edge is lowered in accordance with the extent of the reduction of the shear stress, which results in the reduction of tool wear. It is alleged that, in order to reduce shear stress, manganese sulfide is preferably in the form of oval with a small length-to-width ratio which causes a significant notch effect. All current methods for improving the tool life for continuous-cast free-cutting steel are based on this mechanism. The methods according to this mechanism, however, are not sufficient to meet the industrial machined surface roughness requirements, and the development of novel mechanisms and methods based thereupon are obviously necessary.
As described in Tetsu-To-Hagane, vol. 69, 1983, No. 5, p. 199, since the continuous-cast free-cutting steel in comparison with the ingot mold-cast free-cutting steel produces less fluctuation in the tool life and has a superior homogeniety, a free-cutting steel comparable or superior to the ingot mold-cast free-cutting steel can be expected if the machined surface roughness is improved. Therefore, the development of such a continuous-cast free-cutting steel is greatly desired in industry.
Further, the evaluations of the machined surface roughness do not correspond to each other regarding the practical use of and the results obtained by the established test methods, such as the JIS-method, etc., which have significantly obstructed the development of the continuous-cast free-cutting steel. The present inventors and others, as reported in Tetsu-To-Hagane, vol. 71, 1985, No. 5, s530 (English translation published in Transactions of ISIJ, vol. 25, 1985, No. 9, B227), have already thrown light on the cause of the incompatibility between the evaluations and have developed a testing method which can simulate a cutting condition having a practical use. That is, this method adopts a repetition of a short time cutting and a pause, for example, 2 to 4 sec, according to the most usual machining condition such as in plunge cutting by an automatic screw machine of the free-cutting steel, which is distinct from the methods heretofore used, such as the JIS-method, etc., where a long time cutting duration of, for example, 450 sec, is adopted. Such a difference between the duration of the times of cutting in these testing methods causes a discrepancy in the temperature reached by the tool edge (cutting part) during cutting, and therefore gives rise to the inconsistency which has been found between the performance in practical use and the results obtained by the conventionally established testing methods, such as the JIS-method, etc.
The present inventors and others, as reported in Tetsu-To-Hagane, vol. 71, 1985, No. 5, s531 and s532 (English translation is published in ibid B228 and B229), tested various commercial steels for which the machined surface roughnesses were variously evaluated in practical use, by using the above-mentioned developed testing method and metallurgical observation. The test results showed that, first, in the steel exhibiting an excellent machined surface, a layer of manganese sulfide-base inclusion is formed on the tool edge during cutting, which suppresses the adhesion of work to the tool edge and the generation of built-up edge on the tool edge (Tetsu-To-Hagane, vol. 71, 1985, No. 5, s531). On the contrary, in the steel exhibiting an inferior machined surface, adhesion between the work and the tool and a built-up edge growth are prevalent. The built-up edge once formed on the tool edge falls away onto the machined surface, and thus significantly roughens the machined surface. This suggests that the manganese sulfide-base inclusion layer has an essential function of lubrication between the tool edge and the work. Second, the formation of the layer is promoted as the size (mean sectional area) of the manganese sulfide-base inclusion increases (Tetsu-To-Hagane, vol. 71, 1985, No. 5, s532). This suggests that the increase in the size of the manganese sulfide-base inclusion promotes a separation of the manganese sulfide-base inclusion from the steel, and further, increases the amount of layer formed on the tool edge, which further improves the machined surface roughness.
However, in the continuous-cast free-cutting steel, a condition sufficient to stably form the above-mentioned effective layer of manganese sulfide-base inclusion on the tool edge has yet to be discovered.